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Hoffman et al. [761] studied 45 patients with SLE and showed that sFLC concentrations were approximately 3-fold greater than normal (Figure 35.4). Predictably, sFLC concentrations were higher in SLE patients with renal involvement than in those with normal renal function (Figure 35.5). Clinical scores of SLE correlated with sFLC levels, particularly when the disease was active. In a subsequent prospective study, the clinical scores (European Consensus Lupus Activity Measurement, ECLAM [763]) in eight patients were compared with a variety of laboratory parameters [764]. sFLC concentrations showed a strong correlation with disease activity that was not observed for CRP or erythrocyte sedimentation rate (ESR). A larger study with 75 SLE patients [765] also showed a strong association between ΣFLC concentrations and disease activity (as measured by the SLE Disease Activity Index, SLEDAI; p<0.001). Similar findings were reported by Garcia de Veas Silva et al. [1124]. The association of ΣFLC with disease activity was further corroborated in a study that employed a third disease activity scoring system (British Isles Lupus Assessment Group index, BILAG) [766].

Assessment of autoantibody production is included (directly or indirectly) in SLE disease scoring systems and ΣFLC have, unsurprisingly, been found to be higher in anti-DNA positive versus anti-DNA negative patients (n=62) [767]. These findings were confirmed by Garcia de Veas Silva et al. [1124]. However, in a group of 11 patients [768], falling concentrations of FLCs were monitored after B-cell depletion with rituximab and showed a significant correlation with total IgG but not with anti-DNA titres.

In addition to renal function, a further complicating factor when interpreting sFLC concentrations in SLE patients is the potential influence of immunosuppressant treatments. In a cross-sectional study of 77 SLE patients, Jolly et al. [769] found that the correlation of sFLC concentrations with disease activity (Physicians Global Assessment [PGA] and SLEDAI scoring) was stronger after controlling for steroid use. The authors concluded that sFLC measurement was potentially superior to anti-DNA antibody or IL-6 for disease monitoring.